A server which encapsulates a piece of mutable state. Its StateMsg type specifies which messages it accepts, which messages return a response, and what type that response is.

data StateMsg s res where
  GetState :: StateMsg s s
  GetsState :: (s -> a) -> StateMsg s a
  PutState :: s -> StateMsg s ()
  ModifyState :: (s -> s) -> StateMsg s ()

state :: s -> Server (StateMsg s) ()
state s0 = do
  stateIORef <- liftIO $ newIORef s0

  forever $ receive >>= handle \case
    GetState ->
      liftIO $ readIORef stateIORef

    GetsState f -> do
      s <- liftIO $ readIORef stateIORef
      pure (f s)

    PutState s ->
      liftIO $ writeIORef stateIORef s

    ModifyState f ->
      liftIO $ modifyIORef stateIORef f